Dryer screens in paper machines are used to pass a paper web to be dried through the drying section and support it so that there are as few wire-marks in the finished fibre web as possible, while the permeability and behaviour of the screen within the drying section are such as desired. One typically aims to provide the dryer screen with a surface structure having the greatest possible evenness and woven as closely as possible, which is described by a percentage value called warp fill. In practice, a 100% warp fill is aimed at, though even higher values are theoretically possible. Due to the location of dryer screens, they are subject to varying conditions, being thus exposed to both hot and wet conditions and hot and dry conditions. To operate properly, the dryer screen must have a good dimensional stability and resistance to longitudinal strains in the above-mentioned conditions as well as flexibility. Various warp threads and weave patterns have been used in the manufacture of dryer screen structures. For instance, flat warp threads have been used to achieve desired dryer screen structures. One such structure is disclosed in FI Patent Application 783268.
FI Patent 81858, in turn, discloses a dryer screen comprising two superimposed gauze structures bonded together. The bottom gauze consists of a stiffer, more robust monofilament thread while the surface layer consists of hollow fibres or softer fibres treated with various foam particles. The surface layer and the bottom layer are bonded together by interweaving the weft threads of the bottom layer and the warp threads of the surface layer. A drawback of this structure is that when the hard round weft threads of the bottom layer and the hard round warp threads passing over them are interwoven into the soft surface layer, unevennesses will occur and possibly also wiremarks as the structure of the bottom gauze is stiffer and harder than that of the surface gauze.
In wet screens intended for a different purpose, a high water permeability is required for removing water from the fibre suspension. In such wires, double gauze structures have also been used in which the upper and lower gauze layer are interconnected by passing different warp threads from one layer to the other. A drawback of such structures is that when threads are passed from one layer to the other, problems are usually caused in weaving and wiremarks will occur as such threads disturb the weaving of the other layer. Such structures are disclosed e.g. in FI Patent Application 871230, FI Patent Application 793140, EP Patent Application 116945 and FI Patent Application 893301. To avoid the problems associated with these structures, FI Published Specification 70947 suggests a paper machine wet screen in which the surface gauze and the bottom gauze are bonded together by separate bonding weft threads passing in the transverse direction of the wire, the warp threads of both gauze layers passing around the bonding threads at suitable intervals. Although this structure is operative as such, it is difficult to weave and requires that the feed amount of warp threads can be adjusted in weaving each layer, which requires a complicated weaving machine. Nor can wet screen structures be used as such in dryer screens as their use and desired properties are very different and even opposite in many respects. Accordingly, it is not self-evident that the wet screen structures are applicable as such nor have there been any attempts to use them in the manufacture of dryer screens.